Superior Canal Dehiscence

Symptoms of SCD occasionally overlap with other auditory and vestibular disorders, such as otosclerosis, Meniere’s disease, benign paroxysmal positional vertigo (BPPV), and other forms of perilymphatic fistula (Mechant, Rosowski & McKenna, 2007; Minor, 2000; Smullen, Andrist & Gianoli, 1999) Differential diagnosis of SCD requires careful correlation of case history information, symptomology, comprehensive audiological and vestibular test batteries, and confirmation with medical imaging (high-resolution high resolution computerized tomography [CT] scan of the temporal bone).

Case History

A thorough hearing and balance history should be obtained from the patient. An awareness of the unique auditory and vestibular red flags reported in cases SCD can play a valuable role in guiding the assessment and in differential diagnosis. Below are key questions the audiologist can ask the patients in cases of suspected SCD.

  • Do certain events/stimuli precipitate symptoms? If so, what are they?
  • Noise triggers: Patient complaints may be associated with loud music, being in a noisy environment such as a sports event, sounds on the telephone such as the dial tone or a busy signal, and/or sounds made by the patient such as singing at certain pitches.
  • Pressure triggers: Patient complaints may be associated with pushing on the outer aspect of the ear, blowing through the nose while pinching the nostrils, and/or straining while lifting heavy objects.
  • Are you unusually sensitive to internal body sounds, such as the sound of your own voice, heartbeat, heel strike, etc.?
  • Do you have chronic imbalance?

Audiologic Evaluation

Audiologists typically perform a battery of audiologic evaluations. Below is a list of evaluations that are typically included in this battery and the results that might lead to an SCD diagnosis.

Pure-Tone Audiometry

Purpose: Identify and classify existence, type, and degree of hearing loss

Procedure: Standard behavioral pure-tone audiometry via air and bone conduction

Results: No one pattern of hearing loss defines SCD; rather a variety of abnormalities may appear on the audiogram, including

  • air-bone gaps of 10 dB or greater at lower frequencies (Minor, 2005),
  • better-than-normal absolute bone conduction (lower than 0 dB HL) (Banerjee, Whyte & Atlas, 2005; Hagr, 2010),
  • mild-to-moderate hearing loss in the affected ear.

Immittance Testing

Purpose: Rule out middle ear dysfunction in patients with conductive hearing loss

Procedure: Standard tympanometry with a 226 Hz probe tone and acoustic reflex threshold testing at 500, 1000, and 2000 Hz

Results: Tympanometric peak pressure and compliance are normal; acoustic reflexes are normal, although pressure changes may evoke vestibular symptoms (Carey, Minor & Nager, 2000; Minor, 2005). Acoustic reflexes are often present with air-bone gaps.

Transient Evoked Otoacoustic Emissions (TEOAE)

Purpose: Assess the integrity of the cochlea noninvasively using moving time window analysis (MTWA), which enables determination of the TEOAE duration and more accurately analyzes cochlear amplifier nonlinearities (Thabet, 2011)

Procedure: Standard wideband TEOAE protocol followed by off-line MTWA

Results: Duration of TEOAE is significantly lower than normal (Mechant, Rosowski & McKenna, 2007) (normal duration is ≥8 ms) (Pytel & Büki, 1995).

Electrocochleography (ECOG)

Purpose: Measure the electrical responses of the cochlea and eighth nerve in response to acoustic stimulation

Procedure: Standard clinical ECOG protocol (transtympanic or extratympanic)

Results: Summating potential (SP) is elevated relative to action potential (AP) ratio (SP/AP > 0.4) (Adams, et al., 2011; Arts, Adams, Telian, El-Kashlan & Kileny, 2009).

Vestibular Evaluation

Vestibular Evoked Myogenic Potential (VEMP)

Purpose: Assess vestibular function— specifically the ipsilateral sacculo-collic reflex (cervical VEMP) and the crossed utriculo-ocular reflex (ocular VEMP)—by air conducted auditory stimuli

Procedure: Cervical VEMP (responses measured at the sternocleidomastoid muscle) or ocular VEMP (responses measured at the inferior oblique muscle of the eye)

Results: Thresholds are lower than normal and amplitudes greater than normal for both cervical and ocular VEMPs (Mudduwa, Kara,Whelan & Banerjee, 2010; Welgampola, Myrie, Minor & Carey, 2008).

Dix-Hallpike Maneuver

Purpose: Assess peripheral vestibular function in order to rule out BPPV

Procedure: Begins with patient in sitting position with head turned 45° right or left; moves rapidly to supine head hanging;. after nystagmus subsides, patient returns to sitting position; procedure repeated to determine fatigability of response

Results: Results are negative for BBPV (positive response would be transient, fatiguable, torsional–vertical nystagmus with delayed onset).

Valsalva Maneuver Against Closed Nostrils or Closed Glottis

Purpose: Observe if pressure changes induce nystagmus and/or vertigo

Procedure: Requires patient to take a deep breath, hold the nose, and blow forcefully against pinched nostrils OR take a deep breath and try to blow forcefully against closed glottis

Results: Patient may experience subjective vertigo/disequilibrium and/or nystagmus (downbeating and torsional beat in the direction of the affected ear for closed nostrils and upbeating and torsional beat away from the affected ear for closed glottis) (Carey, Minor & Nager, 2000; Pfammatter, et al., 2010).

Caloric Testing

Purpose: Assess peripheral vestibular function

Procedure: Standard clinical caloric irrigation protocol using air or water

Results: Patient with a large dehiscence may show reduced vestibular function on the affected side (Cremer, Minor & Carey, 2000).

Additional Vestibular Tests

Videonystagmography (VNG), frenzel goggles, tragal compressions, and pneumatic otoscopy may all cause eye movements in the plane of the superior canal as well as concurrent dizziness.

Confirming Diagnosis

If SCD is suspected from case history and audiologic/vestibular test battery, the patient should be referred to an otolaryngologist to confirm the diagnosis. SCD is confirmed by CT scan of the temporal bone is used to confirm the presence of a dehiscence, but audiological studies are needed to confirm that the dehiscence is active as it can be closed by dura.

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